Tek 2215. Vertical Amplifier Issues.

[M3VUV51]

q350 base voltage goes from +.3v pos 1 pot fully ccw to _.3v fully cw 360 base reads -.274 ccw and .455v cw neather chang moving pos 2 pot only pos 1 knob

[dave cox]

(the other pos control does nothing as you probably have the scope in single channel mode)


That looks good. Drive to Q350 / Q360 looks fine.
That leaves Q360, Q386 and associated parts.

Check Q360 emitter (+0.8) and Q386 base (-4.3) while twiddling the pos !

dc

[M3VUV51]

q360 emitter reads ccw -4.761v cw =-1.45 q 386 ccw=-4.84v cw -5.79v

[dave cox]

Is that actually Q360's collector voltage ?

It might be easier to measure the voltage ACROSS a few resistors
You may need clips for this.

R350/R360 while twiddling the POSITION (as before).

R370/R380 while twiddling the A/B SWEEP SEPARATION
( should do nothing, but we are looking for a fault )

dc

[M3VUV51]

q360 collector reads ccw -4.761 cw = -1.45
q360 emmitter= cw .47v ccw 1.125v
r350=cw=.001 ccw=3.989
r360ccw=0.173 cw= 4.353
both measure 221 ohms.

[M3VUV51]

heres some more measurments,at the junction of r354 and 364,i get 8.4v (should be 8.6v)
q360 emitter reads 1.129v,q350 emitter reads 0.574v both should read 0.8v !

[dave cox]

I think Q350/360 are working fine
If you twiddled the position pot just right you would get the emitter voltages of Q350/360 equal and approx the 0.8V specified. The measurements on R350/360 confirm this.

This points the finger at Q386 (previously tested?) but I would check the voltage across R370/380 first. They should be equal ...

There are 2 circuits driving the output stage Q376/377/386/387, the main drive is from Q350/360 (which seem to be working!) and Q370/380 that cause the trace to move vertically when the 'A/B sweep separate' activates - this needs to be eliminated as a possible contributor.

Another thing is that the output stage probably runs 'hot' in order to extract the best performance from the transistors. A simple tester may not stress them enough for the fault to reveal itself.

dc

[M3VUV51]

Firstly thanks for the help dave,over r370 i get 2.11v and over r380 i get 0.7v,this dont seem to alter when i move the pos pots,cheers Paul.

[Top Cap]

I last repaired/re-calibrated one of these nearly 30 years ago but in my notes I did not come up against a fault in the section you are interested in. I did however come across a 2215 and a 2235 with similar problems in their power supplies that may be of help/interest if your scope should have similar problems.

Tests I made on faulty power supply of a 2235.

Both pre-regulator FET Q9070 and its driver Q908 were blown to pieces.
After removal of these devices, a power supply was connected across C940, Positive connected to the lug which has print traveling to R945 and C943.
While carefully watching the current that is being drawn as the voltage was slowly increased from 0V, it was possible to power up the scope to a final power supply setting of +40V at 0.8A.
With the external power supply current limit set to just before limiting, the supply voltage was increased to approximately 52V when the crowbar operated (Q935), sending the remote power supply into fold back current limiting. Thus it was possible to prove basic oscilloscope operation and the over voltage protection circuit.
The external DC supply was now applied to the cathode of diode CR920 in order to prove Q930, Q928 and the pulse width modulator chip U930.
However, the supply tried to power up the main inverter so it was necessary to remove the control chip U930. With a Multimeter connected to pin 12 of the vacant I.C. position, the DC supply was increased from zero. The Multimeter climbed slowly then suddenly jumped to +14V. This action could only be repeated by first removing the supply and constituted a reasonable test of the circuitry around Q930, Q928 and the Zener diode VR925.
After checking the remaining diodes CR920, CR908 (FOUND SHORT CIRCUIT), it now remained to replace the chip U930 and blown transistors prior to re-applying the mains supply.

Model 2215

This fault appeared to be similar to the 2235 above. Internal circuit board appeared identical.
The FET appeared OK with a resistance check. Investigation showed two diodes in the bridge rectifier had gone short circuit, these being CR904 and CR902 on the negative side of the bridge.
U930 and the FET were removed so that the 0 to 40V test could be done using a remote power supply connected across C940. After adjusting the front panel controls it was found that the oscilloscope was working OK. So as before for the 2235, positive of the remote supply was connected to CR920 cathode. The slow start circuit can now be checked (an AVO is best used for observation) where a slowly rising voltage should be observed until 14V is reached, when there is a quick upward swing. This oscilloscope however showed no swing and Q930 was found very ill. This TEK 151-0164 is a 2N2907A PNP Transistor and after replacement the slow start operates.
Replaced U930 and monitored AC current whilst oscilloscope was powered up on a Variac.
The +40V rail measured as +42.5V and the oscilloscope was working.
NOTE:
Use only sufficient voltage to check the Slow Start circuit, it is not a regulator and a maximum of 15V is recommended. The diodes were 1A rated and were replaced with 1N4007 as they have a higher PIV rating.

[M3VUV51]

luckily my psu seems ok,i changed the riffa caps for 8kv rated x types,but managed to miss one under the mains lead socket on the main board,that went bang when testing transistor voltages on the vert amps,it blew the main fuse,i didnt have a 0.01uf cap but had a bunch of 1nf and a 3.3 nf 3kv x type so ended up 5x the 1nf caps in parellel with the 3.3nf cap shrink wraped them all and connected them to the main pcb with flyleads,looks messy but does the job and its all i had to hand!,if i sort the vert amp issue i will order proper replacements.,cheers m3vuv.

[M3VUV51]

this is how q386 tests on my component tester out of circuit.

[M3VUV51]

Well theres good news and bad news on this one,after subbing the duff transistor,i was replacing the crt,i found the horiz deflection plates a pig to get to so decided to extend the wires so i could connect them before pussing the crt into its socket,i had the scope on its side to do this as it was better to solder the wires back into the pcb,i then put the scope normal way up and fitted the crt,ok time to test,on power on after a few seconds smoke came from under the crt,i quickly unplugged it from the mains and turned it over to look at the underside of the pcb where sat a small coil of soler wire that had shorted out the underside of the pcb,powered it up againe to see both traces now sitting about 3/4 up the screen,its the oposit of the original fault!,also it sees no input on either channel and the traces only do a horiz sweep of about 4 divisions,i will wait untill the replacement bfr -96s transistors arrive and fit them and test againe but i feel its not looking good, angry over making sutch a stupid mistake!! :palm:

[PJL]

Originally Posted by M3VUV51

Firstly thanks for the help dave,over r370 i get 2.11v and over r380 i get 0.7v,this dont seem to alter when i move the pos pots,cheers Paul.

This looks wrong, the circuit is designed to put equal constant currents through R370 and R380 when not using the A/B separator then when the A/B separator is operating the current balance can be changed with the 'A/B swp sep' pot.

Section 2-6 of the manual shows the sweep options. Can you confirm you have the A button pressed and not ALT which would enable the 'A/B swp sep' pot.

If it is not on ALT then we have a fault to investigate but it could still be a fault elsewhere that is limiting the current through R370 and we will need the collector voltages of Q370 and Q380 to confirm. Can you also try altering the 'A/B swp sep' when measuring one of these voltages and the position pot when measuring both.

[frsimen]

PJL may well have spotted what is wrong in post 53.

A couple of additional tests would be worthwhile trying.

1) Switch the HORIZONTAL MODE switch to A and turn the A/B SWP SEP control to the mid position. Measure the voltage at the collectors of Q377 and Q387. You should be able to make them equal (9V approximately) by adjusting the Channel 1 shift control to somewhere near the mid position (assuming channel 1 has been selected).

2) If test 1 passes, try adjusting the A/B SWP SEP control. It should make no difference to the Q377/Q387 collector voltages when the A/ALT/B switch is in the A position.

If both tests pass, your oscilloscope is working correctly. If not, the fault finding will need to continue.

[M3VUV51]

Just waiting for the replacement transistors q378 q 386 to arriveas the crt is out at the moment againe,dont see much point in further testing until they are fitted.must say tho my hot air station made them a doddle to remove!.

[dave cox]

Ooops

But we've all done it
The only people that don't make mistakes are the people that don't try!

PJL is right, something is up with the 'A/B trace separate function' - it will cause the V-output amplifier to be skewed 'to one side' (or top/bottom since its vertical) - to be investigated once you're back at square one.

I think you might be close to having this fixed.

dc

[Diabolical Artificer]

Nice one Dave, top notch advice. Same goes for other contributions. I've stayed quiet as my understanding of the circuits is nowhere as complete as yourselves.

Andy.

[M3VUV51]

Well the replacement transistors are fitted. I now have the trace for want of a better word adjustable, but now have no sweep! Did I mention I put the 'scope down on a piece of coiled solder wire, it shorted summat on the main board as summat smoked. pulled the plug but too late I think! Can't see any signs of anything being fried though all i get now is two dots on the screen as if in XY mode. Any ideas where to start? Seems I fixed one issue and created a bucket load more, teach me to keep my bench clean!! TIA.

[frsimen]

That's rather unfortunate.

Have a good look to see if there is anything obvious that has burnt. Favourites would be low value resistors. Transistors and diodes can smoke but not show much change to their appearance afterwards. They will probably have been damaged, however. If you know roughly the area of the board that was in contact with the solder coil that would be a good place to start.

Next you should determine what, if anything, works.

Does the horizontal shift work?
Does the timebase run any time setting? Try with different trigger modes and sources.
Does X-Y mode work?

Check the power supply voltages after that.

Above all, make sure that your work area is tidy and free of anything that could cause a short to avoid even more damage!

Report back what you find, others may be able to offer more help once you give a bit more information.

[M3VUV51]

Here's my latest findings, no hoz shift, no tb whatsoever no xy but it does display a dot in xm mode dot doesn't move with inputs, here's the measurements from test points, firstly resistance to ground, specified ohm readings in brackets followed by my readings
TP500 (114) 89
w975 (95) 96
w985 (330) 0.4
w965 (905) 1.1k
w966 (12.5k) 11k
next voltage readings again speced in brackets followed by my readings
TP500(-8.6v) 8.46
W975 (+8.6v) +8.46v
W985 (+5v) +5.063v
W965 (+30v) 29.72v
W966 (100v) 100.5v